When it comes to bacteria, many people have a pretty simple view: Germs are bad, and our lives should be as free of them as possible.
But an alternate idea suggests just the opposite: Germs are a necessary part of a healthy immune system, helping our body's defenses beef up and fight future illnesses. When a person's exposure to germs is decreased, problems may arise.
The idea is called the hygiene hypothesis. For years, scientists have suspected that it played a role in how diseases affect people in the modern hand-sanitized world, but they never had any specific evidence.
But a new study from researchers at Brigham and Women's Hospital in Boston has changed that.
Researchers studied two kinds of mice: One group had been exposed to a normal bacteria environment, and another group that was germ-free. When scientists compared the immune systems of the two groups of mice, they found what they cited as evidence to support the hygiene hypothesis – the mice that had been exposed to microbes had stronger immune systems than the germ-free mice.
Additionally, the germ-free mice had exaggerated inflammation in their lungs and colon, similar to what is seen in humans who have asthma and ulcerative colitis. The researchers found that a particular kind of immune cell, called an invariant natural killer T cell, was particularly hyperactive in these mice.
But all was not lost for the germ-free mice. When the researchers introduced them to microbes in the first few weeks of their lives, their fragile immune systems beefed up to a normal level. But older germ-free mice didn't get this beneficial effect.
The results were published today in the journal Science.
The researchers only investigated mice, not people, but experts said the biologic mechanism they studied was similar in both rodents and humans.
Dr. Richard Blumberg, one of the study's senior authors, said the findings suggest what scientists had suspected for years -- that germs can benefit human health.
"We have co-evolved with microbes for millions of years. There is a very beneficial role for microbes in health," Blumberg said. "What our study now shows is the critical importance of those microbes in the earliest periods of life."
Although we might not realize it, we are living in a microbial world. Bacteria occupy nearly every conceivable surface, including the surface of our skin and inside our bodies. In fact, the human body has nearly 10 times the amount of bacterial cells as human cells.
As far as the hygiene hypothesis is concerned, this abundance of bacteria is a good thing, since it helps exercise our immune system.
"But as we moved into the 1950s and '60s and '70s, and as we in the developed world became progressively hygienic, we began to think that we wish to protect ourselves, particularly fragile newborn babies, from anything that might be microbial," said Dr. William Schaffner, chairman of preventive medicine at Vanderbilt University Medical Center. "And that turns out to be obviously what we might call an overreaction."
Some scientists believe the consequences of our germophobia are very real. As human health and hygiene have improved, the number of people living with asthma, allergies and inflammatory bowel disease has also been rising. For instance, the number of children with asthma doubled from 3.6 percent in 1980 to 7.5 percent in 1995, according to the Centers for Disease Control and Prevention; by 2005, 12.7 percent of children had been diagnosed with asthma at some point in their lives.
These diseases are often the result of a symphony of genes and environmental factors. But many scientists say the higher numbers can be explained, at least partially, by the hygiene hypothesis. Immune systems that are less accustomed to dealing with microbes may have hyperactive reactions, leading to the wheezing of asthma or the abdominal cramping of inflammatory bowel disease.
"As we're taking better care of our food, have less dirt and bacteria in our diet, and our immune system isn't being stimulated by those microbes," said Dr. Allen Meadows, an allergist and spokesman for the American College of Allergy, Asthma and Immunology.
Although an increase in hyperactive immunity may be an unfortunate side effect of living more sanitary lives, experts say we shouldn't throw the baby out with the bacterial bathwater. In general, it is a good thing that human life is generally more sanitary and some people have been able to lessen their exposure to malevolent microbes.
"Smallpox is not good for anyone," Schaffner said.
Blumberg noted that the study's findings don't mean that people should change anything about the way they try to keep germs at bay. Much more research is needed on just how germs benefit the human immune system.
"It's that middle road that we're seeking. Hand washing and reasonable cleanliness are a good thing," Schaffner said. "And let's all remember, many of the bad guys can be prevented through vaccination."